Coating of particles is an important process for modifying particles and the surface properties of the particles. Methods for particle coating include the Wurster process as described in U.S. Pat. Nos. 2,648,609; 3,117,027; and 3,253,944 and more recently in U.S. Pat. Nos. 4,731,195 and 5,085,930 in which particles are fluidized in some manner and the fluidized particles are then spray coated with coating materials dissolved in various solvents or the coating materials are sprayed onto the core particles as a low viscosity melt; spray coating is also done in which the particles and the coating material is passed through a suitable atomizer. One example of this method is shown in U.S. Pat. No. 4,675,140 in which the coating material is a melted polymer. An interesting method is presented in U.S. Pat. No. 5,262,240 in which the coating is effected by mixing the particles with a latex and drying the resulting mixture. In this process a coated aggregate is produced. Well known methods for the coating of particles with thin organic layers can use a surface active agent such as organosilanes or fluorocarbons to modify the surface properties. In this method the particles are soaked in a solution and the surface active agent reacts with the particle. U.S. Pat. No. 4,994,326 is one example of such a process and the materials resulting form the treatment. Finally, a preferred method for particle coating is tumble blending or high shear hot blending. A number of patents describe this process. U.S. Pat. No. 4,233,387 describes a process wherein electrophotographic carriers are treated with thermoplastic resins about 325.degree. F. The resulting mixture is then cooled, ground, to an appropriate size and used to charge toners in a photocopier. U.S. Pat. No. 4,774,139 describes a process for coating paraffin onto thermoplastic hot melt resin. U.S. Pat. No. 4,885,175 describes a method for coating a sweetener with a molten wax, cooling the mixture, and grinding the cooled mass to the desired size. Finally, as an example in this hot melt blending process U.S. Pat. No. 4,135,566 describes the coating of iron powders with a polymer and additives by mixing the ingredients in a high shear mixer at temperatures above the melting point of the polymer coating material.
In all the above cases the processes are lacking in a number of important aspects. In the Wurster-like processes, the methods all involve having a particle that is fluidizable. Typically, this is a particle with at least an average size of 50 microns. Further, in the Wurster-like process if solutions are used as the coating vehicle, the solvent, water or organic solvent, must be removed by drying. This is tedious for water solutions and dangerous for flammable liquids. In direct atomization methods there is the difficulty of separation of the coated and uncoated particles. While U.S. Pat. No. 4,675,140 describes a method for particle separation, this technique is not universally applicable to all materials. There are two problems that hinder the application of hot blending methods, in which the processing temperature is above the melting point. First, the agglomeration is an undesired side effect for this process. For many applications subsequent grinding and classification processing can be difficult or too expensive. Secondly, operation of the process above the melting point does result in higher energy costs, which is always undesirable. U.S. Pat. No. 5,147,722 teaches a method in which coating of the particles can be done below the melting point of the polymer binder. Under the conditions of this process with high shear mixing and high applied pressure, the particles are coated, but a web-like matrix is formed. However, for various compositions agglomeration is not desired and the use of high shear mixers and high pressure adds extra capital and operating cost U.S. Pat. No. 5,236,649 also teaches that particle coating can be done at a temperature lower than the melting point of the coating material. However, as in U.S. Pat. No. 5,147,722, the process requires high shear mixing to obtain a good coating.
The use of coated particles has use in decorative masonry, proponents for oil wells, taste masking in the food and pharmaceutical industries and in the powder metallurgy industry. The powder metallurgy industry has developed metal-based powder compositions, generally iron-based powders, that can be processed into integral metal parts having various shapes and sizes for uses in various industries, including the automotive and electronics industries. One processing technique for producing the parts from the base powders is to charge the powder into a die cavity and compact the powder under high pressures. The resultant green compact is then removed from the die cavity and sintered to form the final part.
Industrial usage of metal parts manufactured by the compaction and sintering of metal powder compositions is expanding rapidly into a multitude of areas. Manufacture of these parts with metal powder compositions provides substantial benefits in comparison to having to use a molten alloy in the manufacturing process. For instance, the metal powder compositions allow for the manufacturing process to proceed with just a high pressure compaction die machine and a sintering oven. The different parts are made by simply replacing the compaction die. Further, there is no need to handle molten alloys.
In the manufacture of such parts, iron or steel particulate powders are often admixed with at least one other alloying element that is also in particulate form. These alloying elements permit the attainment of higher strength and other mechanical properties in the final sintered part. The alloying elements typically differ from the base iron or steel powders in particle size, shape and density. For example, the average particle size of the iron-based powders is typically about 70-100 microns, or more, while the average particle size of most alloying ingredients is less than about 20 microns, more often less than about 15 microns, and in some cases less than about 5 microns. The alloying powders are purposely used in such a finely-divided state to promote rapid homogenization of the alloy ingredients by solid-state diffusion during the sintering operation.
The presence of the different particle size materials leads to problems such as segregation and dusting upon transportation, storage and use. The iron and alloy element powders are initially blended into a homogeneous powder. The dynamics of handling the powder mixture during storage and transfer cause the smaller alloying powder particles to migrate through the interstices of the iron-based powder matrix, resulting in a loss of homogeneity of the mixture, or segregation. On the other hand, air currents that can develop within the powder matrix as a result of handling can cause the smaller alloying powders, particularly if they are less dense than the iron powders, to migrate upwardly. If these buoyant forces are high enough, some of the alloying particles can, in the phenomenon known as dusting, escape the mixture entirely, resulting in a decrease in the concentration of the alloy element.
Various organic binding agents have been used to bind or "glue" the finer alloying powder to the coarser iron-based particles to prevent segregation and dusting for powders to be compacted at ambient temperatures. For example, U.S. Pat. No. 4,483,905 to Engstrom teaches the use of a binding agent that is broadly described as being of "a sticky or fat character" in an amount up to about 1% by weight of the powder composition. U.S. Pat. No. 4,676,831 to Engstrom discloses the use of certain tall oils as binding agents. Also, U.S. Pat. No. 4,834,800 to Semel discloses the use of certain film-forming polymeric resins that are insoluble or substantially insoluble in water as binding agents.
Various other types of binding agents are set forth in the patent literature. Polyalkylene oxides having molecular weights of at least about 7000 are disclosed as binding agents in U.S. Pat. No. 5,298,055. Combinations of dibasic organic acid and one or more additional components such as solid polyethers, liquid polyethers, and acrylic resins as binding agents are disclosed in U.S. Pat. No. 5,290,336. Binding agents that can be used with high temperature compaction lubricants are disclosed in U.S. Pat. No. 5,368,630.
U.S. Pat. No. 5,480,469 ("469 patent") provides a brief review of the use of binding agents in the powder metallurgy industry. The 469 patent notes that it is important to have not only a powder composition that has the alloying powder adhered to the iron-based powder by way of the binding agent, but to also have a lubricant present to achieve adequate compressibility of the powder composition within the die and to decrease the forces required to remove the part from the die. The 469 patent discusses various references that disclose the use of a binding agent in conjunction with a lubricant powder, such as a metal soap, to be blended with the iron-based and alloying powders. This blend is then heated and mixed to melt the binding agent and the lubricant and to bind the alloy powder to the iron-based powder. This mixture is then cooled to form the final composition. The 469 patent discloses an improvement to this type of technology by using a diamide wax as the binding agent whereby a metal soap lubricant is not required.
The presence of a binding agent should not adversely affect the compressibility of the powder metallurgical composition. The "compressibility" of a powder blend is a measure of its performance under various conditions of compaction. In the art of powder metallurgy, a powder composition is generally compacted under great pressure in a die, and the compacted "green" part is then removed from the die and sintered. It is recognized in this art that the density, and usually the strength, of this green part vary directly with the compaction pressure. In terms of "compressibility", one powder composition is said to be more compressible than another if, at a given compaction pressure, it can be pressed to a higher green density, or alternatively, if it requires less compaction pressure to attain a specified green density. If the binding agent has good "internal" lubrication characteristics, it will enhance the compressibility of the powder composition and result in a higher green density at a given compaction pressure.
Therefore, a need exists for a coating process that can provide a simple and inexpensive method for coating a variety of particles. In the powder metallurgical industry a specific need exists for a metallurgical composition that contains the alloying powder(s) bonded to the metal-based powder where that composition can be prepared in a solvent-less process. The binding agent used in the metallurgical composition should function to decrease the amount of dusting and/or segregation of the alloying powder(s) and also not adversely effect the compressibility of the composition.